There has been known a system that includes a NOx storage-reduction catalyst (hereinafter referred to as an “NSR catalyst”) disposed in an exhaust passage of an internal combustion engine as disclosed in, for example, JP-A-2001-271679. The NSR catalyst has a storage function of internally storing nitrogen oxide (NOx) contained in a burned gas discharged from the internal combustion engine and a catalyst function of purifying, for example, NOx and hydrocarbon (HC). When the internal combustion engine is operated at a lean air-fuel ratio, an exhaust gas containing a large amount of NOx is discharged. The NSR catalyst therefore stores the NOx internally to inhibit the same from being released to a downstream side of the catalyst.
The NOx stored in the NSR catalyst is purified at a predetermined timing. More specifically, the above conventional art system performs rich spike that temporarily discharges an unburned component from the internal combustion engine. The NOx of the catalyst is thereby reacted with the unburned component in the catalyst.
When a large amount of unburned component is discharged from the internal combustion engine as a result of the initiation of the rich spike, the exhaust gas released to the downstream side of the NSR catalyst is a stoichiometric environment as long as NOx to be reduced by the unburned component is left in the catalyst. When reduction of NOx stored in the catalyst is thereafter completed, the unburned component is released downstream of the catalyst, so that the exhaust gas changes to a rich environment. The above conventional art system is adapted to detect such a change of the exhaust gas downstream of the catalyst to the rich environment, from oxygen concentration or nitrogen oxide concentration and, at the detecting timing, terminate the rich spike. This prevents performance of excessive rich spike, so that fuel efficiency can be prevented from being aggravated.    Patent Document 1: JP-A-2001-271679    Patent Document 2: JP-A-2000-356125